Multi-Input Multi-Output (MIMO) technology is to improve efficiency in data transmission and reception by using multiple transmitting antennas and multiple receiving antennas instead of a single transmitting antenna and a single receiving antenna.
If a single antenna is used, a receiver receives data through a single antenna path. However, if multiple antennas are used, the receiver receives data through various paths. Accordingly, data transmission speed and data transmission rate can be improved, and coverage can be increased.
In a wireless communication system which uses multiple antennas, there are provided an open-loop multi-antenna system and a closed-loop multi-antenna system, wherein the open-loop multi-antenna system does not use feedback information from a receiver but the closed-loop multi-antenna system uses feedback information from a receiver. According to the closed-loop multi-antenna system, a receiver transmits feedback information of channel status to a transmitter and the transmitter identifies the channel status through the feedback information, whereby throughput of a wireless communication system is improved.
The closed-loop multi-antenna system uses a precoding scheme. According to the precoding scheme, a transmitter processes transmission data using feedback information of channel status, which is transmitted from a receiver, whereby the data is affected by a channel within the minimum range.
Examples of the precoding scheme include a codebook based precoding scheme for improving a signal to noise ratio (SNR) and a quantization precoding scheme for quantizing channel information and feeding the quantized channel information back.
According to the codebook based precoding scheme, if the receiver selects a precoding matrix from a codebook previously shared between the transmitter and the receiver and feeds an index of the selected precoding matrix back to the transmitter, the transmitter modifies transmission data using the precoding matrix which is fed back. Namely, since feeding all kinds of channel information back causes high system overhead, channel information for feedback is quantized to constitute a codebook and an index is allocated to each of precoding matrixes included in the codebook so that the receiver feeds only the index back to the transmitter, whereby system overhead is reduced.
Examples of the codebook based precoding scheme according to the related art include a per user unitary rate control (PU2RC) and SIC-based per user and stream rate control (S-PUSRC).
First of all, the PU2RC will be described. Fourier basis is used as a unitary matrix for precoding, wherein the unitary matrix is extended in accordance with the number of transmitting antennas and then used as a precoding matrix. A column vector em(g) constituting a unitary matrix used in the PU2RC is expressed by Equation 1.
                                          e            m                          (              g              )                                =                                                    1                                  M                                            ⁡                              [                                                      w                                          0                      ⁢                      m                                                              (                      g                      )                                                        ⁢                                                                          ⁢                  …                  ⁢                                                                          ⁢                                      w                                                                  (                                                  M                          -                          1                                                )                                            ⁢                      m                                                              (                      g                      )                                                                      ]                                      T                          ,                                  ⁢                                  ⁢                              w            nm                          (              g              )                                =                      exp            ⁢                          {                              j                ⁢                                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                    n                                    M                                ⁢                                  (                                      m                    +                                          g                      G                                                        )                                            }                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In the Equation 1, M represents the number of transmitting antennas, and G represents the number of precoding matrix groups. Also, n represents an index of a transmitting antenna, g represents an index of a precoding matrix group, and m represents an index of a virtual beam forming pattern, i.e., a column vector.
In case of the S-PUSRC, a switching beam forming vector is used as a precoding matrix as expressed by Equation 2 below.P=[a1a2a2N]ai=[1ejφi . . . ej(N-1)φi]T, φi=kd sin(θi)  [Equation 2]
In the Equation 2, N represents the number of transmitting antenna devices, ai represents a precoding vector, k represents a wavelength, -i represents a steering direction, and d represents the distance between neighboring transmitting antenna devices.
In a closed-loop single user MIMO system, its throughput depends on accuracy of feedback, which is varied depending on data of a precoding matrix fed back from the receiver. If the number of transmitting antennas increases, a size of a codebook increases, whereby data to be fed back increases. Receiving throughput depends on a design of a codebook. Accordingly, it is important to design a codebook so that data to be fed back from the receiver is small and system throughput is excellent. It is also important to design a codebook so that complexity of a receiver is low.